The present invention relates to a method and an apparatus for controlling an operation of turbine plant, and more particularly for preventing a flashing when the load on the turbine is decreased abruptly.
The turbine plant is used widely for the purpose of electric power generation. In connection with the electric power demand, the turbine is not always required to operate with full power, but required to operate with full power in the daytime to meet a large demand for electric power and to stop or operate with partial load in the night time in which the demand for electric power is rather small. Such alternation of start and stop of operation in one day or partial load operation imposes a problem that the flashing occurs in the deaerator or in the boiler feedwater pump when the power is decreased in conformity with a reduction in the load level. Such flashing adversely affects the control of operation of the plant.
The reason why the flashing occurs is as follows. When the load of the turbine is decreased abruptly, the interior pressure in the deaerator, to which the heated steam is supplied from the turbine, is also decreased. On the other hand, when the load of the plant is decreased below a predetermined level, the feedwater pump is stopped and the hot water in the downcomer pipe remains high temperature. Consequently, the interior pressure in the downcomer pipe becomes lower than the saturated vapour pressure corresponding to an inlet temperature, thus the flashing is occurred in the deaerator and the downcomer pipe. It is also experienced that re-starting of the feedwater pump is often failed because the pump suction head is lowered as a result of the flashing.
Although various proposals have been made to overcome the above-described problems, these proposals are confined to control the plant partially, and no attempt has been made to control the whole plant. For instance, Japanese Patent Laid-Open Publication No. 143103/1976 discloses one proposal to prevent an occurrence of flashing in the downcomer pipe connecting a deaerator to the feedwater pump.
When a main turbine is tripped from 100% load, the downcomer pipe is filled with hot water of the same temperature as the hot water in the deaerator on 100% load, so that flashing occurs in the downcomer pipe. According to the proposal, in order to prevent the occurrence of flashing, the hot water in the downcomer pipe is fed to the boiler through a branch pipe upon such turbine trip so as to remove the hot water remaining at the inlet side of the feedwater pump. Accordingly the occurrence of flashing is prevented even when the condensate in the deaerator, the temperature of which has been lowered due to the turbine trip, reaches the inlet side of the feedwater pump.
According to this arrangement, however, the hot water cannot be sufficiently removed from the downcomer pipe through the branch pipe in response to a reduction in the turbine load and, therefore, the temperature in the downcomer pipe cannot be lowered in response to the turbine load reduction. With this countermeasure, it is not possible to perfectly avoid the occurrence of flashing.